1. Field of the Invention
The present invention relates to a radiation image capturing apparatus that captures radiation image information of a subject with a radiation image information detector.
2. Description of the Related Art
In the medical field, for example, a radiation image capturing apparatus is extensively used, which exposes a subject (patient) to radiation emitted from a radiation source and detects and processes the radiation passed through the subject with a radiation detector to obtain radiation image information.
Japanese Laid-Open Patent Publication No. 11-345956 discloses a radiation image information detector that allows acquisition of radiation image information of sufficient quality for proper diagnosis while reducing the radiation dose to the subject.
FIG. 7 schematically shows the configuration of the radiation image information detector 2 disclosed in Japanese Laid-Open Patent Publication No. 11-345956. The radiation image information detector 2 includes a grid 4 for removing the scattered component of the radiation that has passed through the subject, a phosphor plate 6 for converting the radiation that has passed through the grid 4 into visible light, and a sensor substrate 8 for converting the visible light emitted from the phosphor plate 6 into charge information associated with the radiation image information and storing the charge information. The grid 4, phosphor plate 6, and sensor substrate 8 are arranged in this order within a peripheral frame 9. Provided to the side of the sensor substrate 8 are driver ICs 10 for driving the sensor substrate 8, and reading ICs 12 for reading the radiation image information from the sensor substrate 8 driven by the driver ICs 10. The reading ICs 12 are provided with a heat pipe 14 for releasing the heat generated in the reading ICs 12 to the outside of the radiation image information detector 2. Japanese Laid-Open Patent Publication No. 11-345956 also discloses a mechanism for cooling the reading ICs 12 through use of a Peltier element instead of the heat pipe 14.
The output characteristics of an amplifier circuit, which is an electronic circuit, generally vary with temperature. In order to compensate for this temperature dependency, Japanese Laid-Open Patent Publication No. 09-289424, for example, discloses a variable amplifier circuit generating a reference current that varies with temperature. The reference current is used to cancel out the temperature dependency of a pair of amplifier transistors which make up the amplifier circuit. Further, the prior art disclosed in Japanese Laid-Open Patent Publication No. 2005-286776 is configured to cancel out the variation with temperature in the current provided from a differential amplifier circuit to a current mirror circuit.
In the medical field, a radiation image of high accuracy is required for proper diagnosis. The detector disclosed in Japanese Laid-Open Patent Publication No. 11-345956 cools the reading IC 12 with a cooling means that utilizes a heat pipe 14 or a Peltier element to achieve stable readout of the radiation image information by the reading IC 12. In this case, however, since the heat is released only by means of the heat pipe 14, the temperature of the reading IC 12 may still vary with the ambient temperature, which makes the characteristics of the reading IC 12 unstable and hinders the acquisition of a high accuracy radiation image. Further, the space required for the heat pipe 14 causes an increase in the size of the radiation image information detector 2, making the detector 2 difficult to handle and transport. Use of the Peltier element instead of the heat pipe 14 allows the reading IC 12 to remain at a constant temperature irrespective of the ambient temperature. However, heat release and space for the Peltier element itself are still required.
When a compensation circuit that compensates for the temperature dependency of the output characteristics of an amplifier circuit, such as the compensation circuits disclosed in Japanese Laid-Open Patent Publication Nos. 09-289424 and 2005-286776, is used, the characteristics of the compensation circuit have to match the characteristics of the amplifier circuit with high accuracy. However, since the temperature dependency of an electronic circuit is typically nonlinear, high-accuracy temperature compensation is considerably difficult to achieve. Thus, even when such a compensation circuit is applied to a radiation image information detector for detecting radiation image information, there is no guarantee that high-accuracy radiation image information will be acquired.